Question

. Mr. Smiths old Mustang sounds like booming thunder when he starts it up...need apostrophe

Answer 1

I'm assuming you're asking where the apostrophe should go. Apostrophes are for marking possession. In this sentence, the old Mustang belongs to Mr. Smith so it should go after his name:

Mr. Smith's old Mustang sounds like booming thunder when he starts it up.

Hope this helps!

Answer 2

Mrs. Smith's because it's showing ownership